feat: add Anthropic Messages API support (#18)

Adds --llm-provider flag (openai|anthropic) to switch between API formats.

Anthropic implementation:
- POST /messages endpoint
- x-api-key + anthropic-version headers
- System prompt as top-level field (not a message)
- max_tokens: 8192 for response generation
- Parses content blocks [{type: "text", text: "..."}]

Changes:
- llm/client.go: Provider type, completeAnthropic(), doRequest() shared helper
- cmd/review-bot/main.go: --llm-provider / LLM_PROVIDER flag
- .gitea/actions/review/action.yml: llm-provider input + env
- llm/client_test.go: 4 new tests for Anthropic path

Backwards compatible — default provider is still openai.

Closes #18

cmd/review-bot/main.go

@@ -34,6 +34,7 @@ func main() {
 	dryRun := flag.Bool("dry-run", false, "Print review to stdout instead of posting")
 	llmTemp := flag.Float64("llm-temperature", envOrDefaultFloat("LLM_TEMPERATURE", 0), "LLM temperature (0 = server default)")
 	llmTimeout := flag.Int("llm-timeout", envOrDefaultInt("LLM_TIMEOUT", 300), "LLM request timeout in seconds (default 300)")
+	llmProvider := flag.String("llm-provider", envOrDefault("LLM_PROVIDER", "openai"), "LLM API provider: openai or anthropic")
 
 	flag.Parse()
 
@@ -74,6 +75,12 @@ func main() {
 	if *llmTemp > 0 {
 		llmClient.WithTemperature(*llmTemp)
 	}
+	switch llm.Provider(*llmProvider) {
+	case llm.ProviderOpenAI, llm.ProviderAnthropic:
+		llmClient.WithProvider(llm.Provider(*llmProvider))
+	default:
+		log.Fatalf("Invalid --llm-provider %q, must be openai or anthropic", *llmProvider)
+	}
 	if *llmTimeout > 0 {
 		llmClient.WithTimeout(time.Duration(*llmTimeout) * time.Second)
 	}